The technique disclosed in the present application relates to toothed belts for use in power steering systems.
A technique of using a reduction gear with a belt for electric power steering is known in the art. For example, Japanese Unexamined Patent Publication No. S62-004673 discloses a belt reduction gear using a friction type belt, a so-called V-belt. In this belt reduction gear, since power is transmitted by friction, a large tensile force need be applied to the belt to obtain a sufficient friction force. Accordingly, in the case of using the belt reduction gear described in Japanese Unexamined Patent Publication No. S62-004673, no-load torque is increased, and a steering wheel tends to return slowly.
Japanese Examined Utility Model Publication No. H06-049489 discloses a belt reduction gear using a spur toothed belt (toothed belt). In this type of belt reduction gear, power is transmitted as the teeth of a pulley mesh with the teeth of a belt. Accordingly, a large tensile force need not be applied to the belt as compared to the case of using the V-belt, and no-load torque can be reduced.
In the belt reduction gear using the toothed belt, however, specific operating noise is generated at the start and end of engagement between the teeth of the pulley and the teeth of the belt during rotation of the belt reduction gear. This problem can be reduced to some degree by reducing the size of the teeth of the belt. However, reducing the size of the teeth of the belt decreases strength of the teeth. The decrease in strength of the teeth may cause shearing of the teeth when a large force is applied to the teeth, which may cause undesirable engagement of the sheared teeth. Thus, a speed reduction mechanism may be locked, and steering may become impossible. Accordingly, if the size of the teeth of the belt is reduced in order to reduce the operating noise, the belt width need be increased to ensure the strength of the teeth. This increases the size of the reduction gear, making it difficult to place the reduction gear in a vehicle body. Increasing the belt width also causes an increase in noise.
A pulley having helical teeth and a toothed belt having helical teeth that mesh with the helical teeth of the pulley are therefore used as a solution (see Japanese Unexamined Patent Publication Nos. 2005-29145 and 2004-314770). The term “helical teeth” refers to the teeth whose tooth trace is tilted at a predetermined angle with respect to the belt lateral direction perpendicular to the traveling direction of the belt when this term is used for the belt, and with respect to the pulley lateral direction perpendicular to the rotational direction of the pulley when this term is used for the pulley. In the case of using the toothed belt having the helical teeth, the tooth of the belt meshes with the tooth of the pulley continuously from one end of the tooth to the other when the reduction gear is in operation. This can reduce noise that is generated when the teeth engage with each other.